Power Angle Monitor

ABSTRACT

A system and method for monitoring the rotation of a generator rotor. The monitor uses a light beam directed toward the rotor to detect a marking thereon, and generates an electrical pulse when the marking is detected. The time between the pulse and a reference point (such as a zero crossing) of the signal waveform from the terminals of the generator may be used to calculate the power angle of the generator. The system is adaptive in that it can account for new markings on the rotor. The system may be connected to a network so that power angles from various generators on the electrical network may be compared. The system may further be connected to a common time source such that a time stamp may be applied to the power angles from various generators, allowing for more accurate comparison of the power angles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional Applicationentitled “POWER ANGLE MONITOR”, filed 8 Oct. 2008, having Ser. No.12/247,866, which claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Application entitled “POWER ANGLE MONITOR”, filed on 9 Oct.2007, having Ser. No. 60/978,634, naming Edmund O. Schweitzer, III andDavid E. Whitehead as inventors, the complete disclosures thereof beingincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to monitoring the rotation of a generator rotor.More particularly, this disclosure relates to monitoring the rotation ofthe rotor using a light source and light detector for detecting markingson the rotor. This disclosure also relates to calculating the powerangle of the generator and using power angles of several generators in apower system for control, automation, or protection.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the disclosure aredescribed, including various embodiments of the disclosure withreference to the figures, in which:

FIGS. 1A and 1B illustrate cross-sections of a synchronous generator;

FIG. 2 is a power angle curve for an exemplary power generator;

FIG. 3 is a simplified functional block diagram of a generator system;

FIG. 4 is a graph illustrating power angle determination;

FIG. 5 is a graph illustrating power angle determination;

FIG. 6 is a graph illustrating power angle determination;

FIG. 7 is a flow chart illustrating a power angle calculation for agenerator;

FIG. 8 is a simplified one-line schematic diagram of a power system withtwo power generators;

FIG. 9A is a one-line diagram of an electric power system illustratingrelative power angle determination using time-synchronized phasor data;

FIGS. 9B through 9D are graphs illustrating power angle determinationfor several generators; and,

FIG. 10 is a flow chart illustrating power angle calculation for asystem of generators.

DETAILED DESCRIPTION

In an electrical power system information about the power angle(sometimes referred to as a rotor angle) of a generator is beneficialfor providing appropriate power system control, automation, andprotection. In particular, information about the power angle influencesdecisions on power generator levels, load shedding or adding, islanding,joining electrical networks, and so forth. Such changes to the networkmay drive a generator to instability for certain conditions of the powerangle of the particular generator. As a result, to maintain stability,information concerning the power angle of a particular generator isimportant in power system protection, automation, or control.

In some conventional generators, the rotor is monitored using a discinstalled thereon and a reader that reads the angular position of thedisc. For example, the disc may include either apertures or markingsthat are read by the reader to determine the angular position of arotor. However, conventional systems are difficult to install becausethe disc must be installed around the rotor, the rotor must be stoppedto install the disc, and conventional receivers typically do not allowfor changes in the markings on the disc, whether the changes areintentional or not. Because of the disc and the optical readers,conventional systems also take up significant space on the generator.

The power angle monitor disclosed herein may be used with a variety ofpower generators that include a rotor. The rotor angle monitor may alsobe used in power systems that include multiple generators forcoordination of power output, which directly relates to the power anglesof the generators and for various protection, automation, or controlschemes used with the power system.

The embodiments of the disclosure will be best understood by referenceto the drawings, wherein like parts are designated by like numeralsthroughout. It will be readily understood that the components of thedisclosed embodiments, as generally described and illustrated in thefigures herein, could be arranged and designed in a wide variety ofdifferent configurations. Thus, the following detailed description ofthe embodiments of the systems and methods of the disclosure is notintended to limit the scope of the disclosure, as claimed, but is merelyrepresentative of possible embodiments of the disclosure. In addition,the steps of a method do not necessarily need to be executed in anyspecific order, or even sequentially, nor need the steps be executedonly once, unless otherwise specified.

In some cases, well-known features, structures or operations are notshown or described in detail. Furthermore, the described features,structures, or operations may be combined in any suitable manner in oneor more embodiments. It will also be readily understood that thecomponents of the embodiments as generally described and illustrated inthe figures herein could be arranged and designed in a wide variety ofdifferent configurations.

Several aspects of the embodiments described will be illustrated assoftware modules or components. As used herein, a software module orcomponent may include any type of computer instruction or computerexecutable code located within a memory device and/or transmitted aselectronic signals over a system bus or wired or wireless network. Asoftware module or component may, for instance, comprise one or morephysical or logical blocks of computer instructions, which may beorganized as a routine, program, object, component, data structure,etc., that performs one or more tasks or implements particular abstractdata types.

In certain embodiments, a particular software module or component maycomprise disparate instructions stored in different locations of amemory device, which together implement the described functionality ofthe module. Indeed, a module or component may comprise a singleinstruction or many instructions, and may be distributed over severaldifferent code segments, among different programs, and across severalmemory devices. Some embodiments may be practiced in a distributedcomputing environment where tasks are performed by a remote processingdevice linked through a communications network. In a distributedcomputing environment, software modules or components may be located inlocal and/or remote memory storage devices. In addition, data being tiedor rendered together in a database record may be resident in the samememory device, or across several memory devices, and may be linkedtogether in fields of a record in a database across a network.

Embodiments may be provided as a computer program product including amachine-readable medium having stored thereon instructions that may beused to program a computer (or other electronic device) to performprocesses described herein. The machine-readable medium may include, butis not limited to, hard drives, floppy diskettes, optical disks,CD-ROMs, DVD-ROMs, ROMs, RAMs, EPROMs, EEPROMs, magnetic or opticalcards, solid-state memory devices, or other types ofmedia/machine-readable medium suitable for storing electronicinstructions.

Power Angle Determination

Electrical generators are used in electrical power networks to providepower to the electrical network. Several types of generators are used,most of which include a rotor that is caused to rotate by a mechanicalforce or prime mover provided by, for example, wind flow, water flow,steam flow, torque from an engine, or the like. One particular type ofgenerator is a synchronous generator. Synchronous generators are used inelectric utility systems to convert mechanical rotation into alternatingelectric current via well-known electromagnetic principles. Aftersuitable conditioning, the alternating electrical current is typicallytransmitted and distributed to a variety of power system loads withinthe electric network.

As is known, and as illustrated in FIGS. 1A and 1B, synchronousgenerator design is based on Faraday's law of electromagnetic inductionand includes a rotating rotor 204 driven by an external torque forinducing an electromagnetic field (EMF) in a stationary stator. Therotor 204 includes a field winding 158 wrapped around a rotor body, andthe stator includes an armature winding wrapped around an armature body.

In operation, a direct current is made to flow (using, for example, andexciter voltage 160) in the field winding 158 to generate a magneticfield. When the rotor 204 is made to rotate, the magnetic field rotateswith it, passing the stator winding(s) and inducing a time-varyingelectric current therein.

FIG. 1B illustrates a 3-phase synchronous generator operating under theprinciples stated above. The generator includes three windings 153 a to153 a′, 153 b to 153 b′, and 153 c to 153 c′. The stators are eachseparated by 120° such that when the electrical field associated withthe rotor 204 passes the stator, each electrical current induced in theterminal pairs 155 a and 155 a′, 155 b and 155 b′, and 155 c and 155 c′of each stator is separated by 120 electrical degrees. As the poles ofthe electrical field associated with the rotor 204 pass the stators, thevoltage present on the terminals of the stators oscillates, and analternating current results.

The period (T_(I)) of the resulting alternating current from asynchronous generator with N poles, and which has a period of rotationof T_(G) can be calculated using the formula:

T_(I)=NT_(G)

The alternating current from the illustrated generator will have aperiod equal to the period of rotation of the rotor because thegenerator rotor includes a single pair of poles. The present inventionmay, however, be applied to any rotor regardless of the number of phasesor pairs of poles. Thus, the mechanical angular position of the rotor204 is related to the time-varying electrical output of the terminals155 a-155 c. As described below, however, this relationship can beinfluenced by, for example, an electrical load on the terminals of thegenerator. This relationship can be monitored during operation of thegenerator by monitoring the power angle of the generator.

Power angle is the angle between the axis of a generator rotor and theresultant magnetic field axis. The position of the generator rotor axisis a function of the mechanical power input on the generator, whichcauses a torque on the rotor. Under normal operating conditions, andequal but opposite electrical torque is also placed on the rotor due tothe electric output from the generator. In conditions where themechanical torque and the electrical torque fall out of balance, thepower angle may shift or oscillate, depending on the magnitude of theunbalance.

FIG. 2 is one illustration of a power angle curve that shows therelationship between the power angle (δ) and the input mechanical power(P_(m)). Under balanced conditions, the mechanical power input P_(mo)results in a balance point a and power angle δ₀. For a situation wherethe electric torque that balances the mechanical torque decreases (forexample, an increase in the mechanical power or a fault that decreasesthe electric power from the generator), the mechanical torque causes therotor to rotate at an increased rate, causing the power angle toincrease. In FIG. 2, the mechanical power is increased to P_(m0) fromP_(m1), resulting in an increase in the power angle. Though theresultant electrical torque would balance the new mechanical torque atpoint b, the inertia of the rotor would continue to increase the powerangle to, for example, δ_(max) at point c. In a stable system, the rotorwould experience negative acceleration, and eventually come to balancepoint b at power angle δ_(f). For a stable system, the area sweptbetween the power angle curve between points a and b 262 will equal thearea swept between the power angle curve between points b and c. This isthe so-called equal-area criterion. That is, for a generator to remainstable, a disturbance must satisfy the equation:

∫_(δ) _(o) ^(δ)(P _(c) −P _(m))dδ=0

If the step increase of the disturbance is too large, the system couldgo unstable. Knowing the maximum power angle δ_(max) allows operators toknow how much power can be safely produced without causing an increaseddemand on the generator in an unstable condition. As represented in FIG.2, area 262 represents the kinetic energy gained during accelerationbetween points a and b, where area 264 represents kinetic energy lostduring deceleration between points c and b.

It should also be noted that in an improperly damped system, the powerangle may oscillate between points a and c around point b for thedisturbance step change from mechanical power increase from P_(mo) toP_(m1). In typical power systems, however, damping is present in theform of resistances, field flux variations and rotor damper windings,all of which assist in damping the oscillations such that the systemsettles at balance point b.

A. Monitoring the Mechanical Angular Position of the Rotor

A power angle can be determined using information about the angularposition of the rotor and a signal waveform such as from a voltageoutput of the generator. The angular position of the rotor can bedetermined using the system illustrated in FIG. 3. According to thissystem, a rotor 204 rotates about its axis. Present on the surface ofthe rotor 204 are a plurality of markings 206. The markings may includescratches on the rotor 204, either intentionally or unintentionallymade, a reflective marking such as tape or paint, a code attached to therotor such as a bar code, or the like. A light 212 from source 208 isdirected toward the rotor 204 at a particular longitudinal positionalong the length of the rotor 204. The light source 208 may be anycapable of providing a reflected reading from the rotor 204, and mayinclude a laser, a light emitting diode (LED), or the like. As the rotor204 rotates, the markings cause changes in the amount of the reflectedlight from the light source 208, to the light detected by a sensor 216.A sensor 216 receives the reflected light information from the rotor204. The sensor 216 may be capable of receiving the reflected lightinformation from the rotor 204 and may include a photo transistor or thelike. The output of the sensor 216 is transmitted over a link 224 to acontroller 228. The controller 228 produces an analog or digital record237 of the sensor's 216 signal. Thus, a marking profile is created thatcorresponds with the marks detected on the rotor. This marking profile237 is repeated for each complete revolution of the rotor 204 as long asno new markings are added to the rotor. The marking profile 237 isstored within a storage medium 232, which may be analog or digital, butis preferably a non-volatile digital medium such as a FLASH memory or ahard disk. As the controller 228 receives output from the sensor 216 itcompares the output to the marking profile 237 such that any additionalmarkings detected may be added to the marking profile 237.

The pattern repeats identically unless new markings are made. Forexample, a new scratch may appear on the rotor 204. These new markingsresult in a new marking profile 237. Program 232 is adaptable such thatit can recognize and accept new markings on the rotor 204, and updatethe marking profile 237.

Any one of the particular parts of the reference pattern can be selectedfor the reference marking. A digital rotor pulse 236 is generated by thecontroller 228 when the reference marking is recognized during eachrevolution of the rotor. The digital rotor pulse 236 can be used alongwith the voltage of the generator to obtain the power angle of thegenerator.

As described above, a three-phase generator will include three sets ofterminals 352, 354, and 356. Voltage signal waveforms are sampled fromeach of these terminals by the controller 228 using voltage transformers(potential transformers or similar devices) 358 a, 358 b, 358 c.

B. Calculating the Power Angle

FIG. 4 illustrates a signal waveform generated by a generator consistingof a terminal voltage (V_(t)) of one pair of the terminals(corresponding with one phase) of the generator over time. Under no loador an open circuit condition (OCC), the power output of the generatorwill be zero, and the power angle will be zero. Under such conditions,we can take a reference point when the voltage crosses zero, such as areference point t_(ref) 402 (though this embodiment is described interms of a zero crossing, any particular reference point may be used,such as a particular angle on the signal waveform, using the systems andmethods described herein). It should be noted that the reference pointt_(ref) 402 may also be referred to as a reference time. Point t₀indicates the voltage at the point that the rotor pulse 236(corresponding to a particular position of the rotor) is generated bythe controller 228 when there is no load connected to the generator(i.e. OCC). This can be measured, for example, by opening the circuitbreaker connecting the generator to the power system. With the rotor 204rotating at a synchronous speed, the time between the rotor pulse 236and the reference point t_(ref) 402 will be constant during the OCC.Either the positive-slope or negative-slope zero crossing may be usedfor the reference point t_(ref) 402. In one embodiment, it is the same(positive-slope or negative-slope) zero crossing used throughout. Whenthere is a load connected to the generator, however, the position of therotor pulse t₁ with respect to the reference point t_(ref) 402 changes.

As can be seen in FIG. 5, when the generator is connected to a load, therotor pulse is generated at a different voltage, as indicated by t₁. Ineffect, the rotor pulse shifts with respect to the zero-crossing of theoutput voltage of the generator. The shift can be measured using areference point t_(ref) 402 of the output voltage of the generator. Asseen in FIG. 6 and the calculation below, Δ_(p) is the power angle ofthe generator.

With the gathered information, the power angle δ can be calculated usingthe equation:

$\delta = {\frac{\Delta_{1} - \Delta_{0}}{T}*2\pi}$

where T is the period of the rotor, Δ₁ is the time between the rotorpulse t₁ and the reference point 402 when the generator is connected tothe load, and Δ₀ is the time between the rotor pulse t₀ and thereference point t_(ref) 402 during OCC. For example, for a 60 Hz powersystem, T=16.67 ms, and assuming that Δ₀=3 ms and Δ₁=4.5 ms, then thepower angle δ is 0.18π (or 32.4°). Calculating the power angle in thismanner yields an accurate power angle when the frequency of the powersystem does not change between the time when Δ₀ is calculated and thetime that the power angle is calculated. Using the calculated powerangle, operators can make better-informed decisions for power systemautomation, control, or protection.

In the case where the period of rotation is not constant, a compensationmay be made for the change in period by multiplying Δ₀ by a compensationfactor m. The compensation factor may be calculated by comparing the twoperiods:

$m = \frac{T_{1}}{T_{0}}$

where T₀ is the period of rotation for the rotor during the OCCcondition (when Δ₀ is measured), and T₁ is the period of rotation forthe rotor when the power angle is determined (when Δ₁ is measured).

Using the compensation factor m, the power angle can be determined usingthe equation:

$\delta_{p} = {{\frac{\Delta_{1} - {m\; \Delta_{0}}}{T_{1}}2\pi} = {\frac{\Delta_{1} - {\Delta_{0}\left( \frac{T_{1}}{T_{0}} \right)}}{T_{1}}2\pi}}$

Because the period of rotation T is needed to calculate the power angle,the system constantly measures either the frequency or the period of therotor. This may be performed using zero crossings (such as voltage zerocrossings) by, for example, measuring the time between each zerocrossing and multiplying by two or measuring the time between eachnegative zero crossing or between each positive zero crossing. This maybe performed using the physical rotation of the rotor by, for example,measuring the time between each rotor pulse. Further, the period may becalculated using the frequency (the period being the reciprocal of thefrequency), which may be available to the controller 228 from otheroperations or calculations made by the controller. There are furthermethods of calculating the period of rotation, any of which may be usedas needed.

FIG. 7 illustrates a summary of the method that may be used to determinethe power angle of a generator using the disclosed method and apparatus.As described above, the system generates an analog or digitalrepresentation (marking profile) of markings on the selectedcircumferential section of a generator rotor 710. A reference mark isselected from within the marking profile, and a rotor pulse that isassociated with the detection of that reference mark is generated foreach rotation of the rotor 720, and may be time-stamped 725 using thecommon time reference. Throughout, the period of rotation of the rotoris measured for each rotation of the rotor using the rotor pulse. Theperiod may be measured by measuring the time between rotor pulses. Inthe example where the reference pulse and the rotor pulse are timestamped, the time between the reference pulse and the rotor pulse may becalculated using subtraction. Alternatively, a timer may be started wheneither the reference pulse or the rotor pulse occurs, and end when theother of the reference pulse and the rotor pulse occurs, resulting inthe time between the rotor pulse and the reference pulse. During OCC, atime between the rotor pulse and a reference point (eitherpositive-slope or negative-slope) is measured 730, and the period ofrotation for this time is measured and recorded. Alternatively, and asdiscussed in conjunction with FIG. 9A, a reference point on a signalwaveform other than a zero crossing (such as a sample time) may be used.The generator is then connected to a load such that power is generated740. The rotor pulse and the zero crossing (or other reference point ona signal waveform such as a sample time) are time stamped 745. The timebetween the rotor pulse and the zero crossing (the same-sloped zerocrossing as used in step 730) is measured 750, and the period ofrotation is also measured and recorded. As above, in the example wherethe reference pulse and the rotor pulse are time stamped, the timebetween the reference pulse and the rotor pulse may be calculated usingsubtraction. Alternatively, a timer may be started when either thereference pulse or the rotor pulse occurs, and end when the other of thereference pulse and the rotor pulse occurs, resulting in the timebetween the rotor pulse and the reference pulse. The power angle δ_(p)is then calculated using the recorded times and periods of rotation 760.As described below, the power angle may be time stamped with a commontime and shared across a network 770. Further, the time-stamp of therotor pulse, the time stamp of the zero crossing (or other referencepoint on the signal waveform) may also be shared over the network.

The method described above essentially measures a “shift” (or rotationor angular change) in the position of the rotor pulse in relation to thereference point. That is, when the generator begins to generate power(i.e. a load is connected to the generator), the electrical anglebetween the rotor pulse and the reference point shifts. The shift can beused to determine the power angle of the generator as it generates powerusing the above equation. Thus, an optical rotor monitor is used todetermine a shift in the position of the rotor pulse in relation to thereference point such that the power angle of the generator may becalculated and monitored.

C. Power Angles on an Electrical Network

As mentioned above, the power angles of a plurality of generators on anelectrical power system network can be beneficial in making power systemprotection, control, and automation decisions. A simplified network isillustrated in FIG. 8, which depicts a power network 800. A firstgenerator 110 produces an internal excitation voltage E₁ and operates ata power angle δ₁ as measured at the rotor of the generator 110. Thefirst generator 110 has an internal reactance X₁ 108 in series with thevoltage V₁ measured at the terminals 112 of generator 110. A secondgenerator 120 produces an internal excitation voltage E₂ and operates ata power angle δ₂ as measured at the rotor of the generator 120. Thesecond generator 120 has an internal reactance X₂ 128 in series with thevoltage V₂ measured at the terminals 132 of the generator 120. A networkof load impedances 116 designated X_(L) is powered by both generators.Connection of generators 110 and 120 to a load is well known in the art.

Both generators may have different stable operating ranges; i.e., theirpower angle curves (as illustrated in FIG. 2) may differ. Whenadditional loads are switched onto the local electrical network, caremust be taken to ensure that all affected generators will remain stable.

Turning again to FIG. 3, a plurality of generators 102 a-102 n areconnected to the communications network 308 in a similar manner. Forexample, controller 228 is connected to the network 308 such that thepower angle of the generator associated with the rotor 204 may becommunicated over the network. Likewise, the power angles of generators102 a-102 n are communicated over the network. Power angles from variousgenerators on the electrical power network can then be gathered using acomputer 321 connected to the network over link 310.

A comparison of the various power angles from the plurality ofgenerators may then be made. Decisions of whether to add or shed loads,connect or disconnect generators, island a portion of the electricalnetwork determine how close a generator is to its stability limits, andthe like may be made depending on the power angles of the variousgenerators. Power angles may also be used to determine a generatorcapacity, which is beneficial in deciding whether more load could beadded. For example, if adding more load to the unconnected generators104, 124 would be detrimental to the electrical network or generators,appropriate decisions could be made. Further, if a generator is to beconnected to a loaded electrical network, the power angle of thegenerator may be compared with power angles of the generators connectedto the electrical network to ensure that the connection of theadditional generator would benefit the electrical network.

The network to which the generators and computer are connected may beany of various types available such as, for example, the world-wide web,a dial-up network, a local area network (LAN), a wide area network(WAN), and the like. Communication over the network may be made overvarious available means such as, for example, over fiber optic lines,metal cables, radio transmission, and the like. Communication over thenetwork may follow one of various known protocols such as, for example,IEC-61850, Modbus, DNP3, SEL Fast Message, Mirrored Bits® (availablefrom Schweitzer Engineering Laboratories, Inc, Pullman, Wash., and asdescribed in U.S. Pat. Nos. 5,793,750 and 6,947,269, incorporated hereinby reference), IEEE C37.118, and the like.

For the comparison of the various power angles from the plurality ofgenerators to be more accurate, each power angle may be associated witha time stamp (“time stamped”) of a common time reference. The commontime reference may be available to controllers such as controller 228 sothat the controllers can associate a time stamp to the calculated powerangle from each generator. Further, the controller 228 may assign a timestamp with the rotor pulse and with the zero-crossing such that thistime-stamped data may be shared over the communications network. Thecommon time reference is a time reference that is common to the deviceson the network such that all may either operate at coordinated timesand/or stamp communications with a common time for proper comparison andordering of the communications. The common time reference may beprovided to the devices via, for example, a global positioning system(GPS) using an IRIG-B format. FIG. 3 illustrates a system where thecommon time reference is provided using GPS through a GPS receiver 220connected to the controller 228 using line 304.

The computer 321 may include software that allows the user to visualizethe network as well as the power angles of the plurality of generatorswhether or not connected to the network. The software may also beadapted to allow a user to control certain aspects of the electricalpower network such as opening or closing circuit breakers (to add orshed loads, or cause islanding of certain portions of the electricalpower network, for example), connecting or disconnecting the generatorsfrom the electrical power system, change exciter voltage, affect taps ofa voltage regulator, operate a capacitor bank, and the like.

FIGS. 9A through 9D illustrate an embodiment of the present inventionused to determine the relative power angles of several generators on anelectrical power system 900. The power system 900 includes variousgenerators 902, 904, 906, and 908 connected to a common power system910. Each generator is interconnected via conductors 910, 916, and 918and/or a bus 912, 914. In this system, instead of measuring the shift ofthe rotor pulse as it relates to a zero crossing, the shift is measuredagainst a common time-referenced sample.

In particular, as can be seen from FIG. 9A, a sample waveform diagramand phasor are shown for each generator. The waveform and phasor forgenerator 902 is illustrated at 903, those for generator 904 isillustrated at 905, those for generator 906 is illustrated at 907, andthose for generator 908 is illustrated at 909. As samples are taken fromthe waveforms corresponding to each generator, the phase angle andmagnitude are calculated, the time between the sample time and the lastrotor pulse is determined, and this data is time-stamped andcommunicated to, for example, computer 321. The time-stamped data fromeach generator 902, 904, 906, and 908 are then time aligned such thatthe phase angle and magnitude and time between the sample time and thelast rotor pulse is determined can be compared. For example, as shown inFIG. 9A, this data for a particular instant in time is illustrated andcan be compared.

The phasor and waveform data 903 from generator 902 taken at referencetime 954 indicates a particular phase angle and magnitude as well as atime between the last rotor pulse 952 and the reference time 954.Likewise, the phasor and waveform data 905, 907, and 909 from generators904, 906, and 908 taken at reference time 954 indicate particular phaseangles and magnitudes as well as times between the last rotor pulse 956,960, and 964 and the reference time 954, respectively. This data may beused to determine the power angles of each individual generator as wellas relative power angles between generators. That is, for each generatorduring OCC the time between a reference time 954 and the rotor pulse 952is measured (Δ₀). When load is added, the time between the referencetime 954 and the rotor pulse 952 is again measured (Δ₁). As describedabove, Δ₀ and Δ₀ can be used to determine the power angle of eachgenerator. The power angle of each generator may be compared with thepower angles of certain of the other generators such that the relativepower angles between each of the generators may be monitored.

Turning to FIG. 9A, to determine the power angle of generator A 902, theOCC time between the rotor pulse and a particular phase angle may becalculated. When load is added to generator A 902, the relative shiftbetween the rotor pulse and the particular phase angle can be used todetermine the power angle of generator A 902. That is, instead of usingthe time of the zero crossing as described above, another part of thesignal waveform may be used.

Likewise, the relative power angles between generators may be monitoredusing the system and methods described herein. As described above, FIG.9A illustrates an electric power system 900 including generators A, B,C, and D 902, 904, 906, 908. Using the time-stamped phasor and rotorpulse time information from each generator, relative power anglesbetween the generators may be determined. For example, a relative shiftbetween the rotor pulse 952 of generator A 902 and a particularreference point on the signal waveform from generator 908 may bedetermined. The reference point may be a zero crossing, the point of aparticular angle, or the like. Using the methods described above, theshift may be used to calculate an angle between the rotor pulse 952 andthe point on the signal waveform from generator 908. This angle may bemonitored and/or used in protection, control, automation or metering ofthe electric power system 900.

Calculating the power angles and relative power angles of the variousgenerators of the electric power system 900 is illustrated in FIGS. 9Bthrough 9D. As with FIG. 4, FIG. 9B shows an OCC. However, the signalwaveforms of each of generators A, B, C, and D are shown. The referencetime 920 is provided, and the time of each of the rotor pulses t₀ _(—)_(A) 922, t₀ _(—) _(B) 924, t₀ _(—) _(C) 926, t₀ _(—) _(D) 928 isrecorded. The time differences Δ₀ _(—) _(A) 930, Δ₀ _(—) _(B) 932, Δ₀_(—) _(C) 934, and Δ₀ _(—) _(D) 936 between the reference time 920 andthe rotor pulses t₀ _(—) _(A) 922, t₀ _(—) _(B) 924, t₀ _(—) _(C) 926,t₀ _(—) _(D) 928 are calculated for each generator. In FIG. 9C, load isadded to each of the generators and the time differences Δ₁ _(—) _(A)930, Δ₁ _(—) _(B) 932, Δ₁ _(—) _(C) 934, and Δ₁ _(—) _(D) 936 betweenthe reference time 920 and the rotor pulses under loaded conditions t₁_(—) _(A) 922, t₁ _(—) _(B) 924, t₁ _(—) _(C) 926, t₁ _(—) _(D) 928 arecalculated for each generator.

Power angles of each of the generators can be determined over time usingthe newly generated time differences Δ_(P) _(—) _(A), Δ_(P) _(—) _(B),Δ_(P) _(—) _(C), and Δ_(P) _(—) _(D). Over time, the relative powerangles between each of the generators may be monitored, and protectionor control decisions may be made depending on the relative power angles.For example, load shedding decisions may be made depending on therelative power angles between the generators.

Though particular methods of sharing power angles over a network aredescribed, various others are possible using the system and methodsherein. For example, in FIG. 9A the power angle between variousgenerators may be compared as long as the rotor angle monitor of eachgenerator has a particular time reference. For example, that timereference may be a Start of Second available from the common time source(such as, for example, a particular bit of an IRIG-B time signal from aGPS). Computers connected to each generator could measure the timebetween the rotor pulse and the second of century (as defined, forexample, in IEEE Standards 1344 or 1588, herein incorporated byreference in its entirety), and compare the relative shift of this timebetween generators. The comparison is performed by sharing time-stampedinformation between the generators (using, for example, time-stampedphasor data) as described herein.

It should be noted that there are several available reference times insuch a network. In one example, a reference time 954 corresponds with acommon time (such as may be available from a GPS, as discussed furtherherein). The common time may provide a reference time signal such as atop-of-second or second-of-century time signal throughout the network,either of which may be used as the reference time. Another availablereference time is a sample signal that may be provided by a monitoringdevice on the network. In such an example, the monitoring deviceprovides a signal indicating that such samples are to be taken. Further,the reference time may be a zero crossing of a sampled waveform from oneof the generators. Likewise, the reference time may be a particularphase angle of the signal waveform from a particular generator. Thereference time may be a rotor pulse from one of the generators.

FIG. 10 illustrates a flow chart of a method for determining the powerangle of generators on an electric power system. As described above, thesystem generates an analog or digital representation (marking profile)of markings on the selected circumferential section of a generator rotor1010 for each generator. For each generator, a reference mark isselected from within the marking profile, and a rotor pulse that isassociated with the detection of that reference mark is generated foreach rotation of the rotor 1020, and may be time-stamped using thecommon time reference. For the system, a reference time or referencepoint is selected 1025. As described above, a number of particularreference times or points are available such as the zero-crossing of asignal waveform from a particular generator, a particular sample time, arotor pulse from a particular generator, a particular repeating point intime from a time source, a particular angle of a signal waveform from aparticular generator, and the like. Each occurrence of the referencetime or reference point may be time-stamped. Each generator may have itsown unique reference time such as a zero crossing from each generator.

Throughout, the period of rotation of the rotor is measured for eachrotation of the rotor using, for example, the rotor pulse. The periodmay be measured by measuring the time between rotor pulses. In theexample where the reference pulse and the rotor pulse are time stamped,the time between the reference pulse and the rotor pulse may becalculated using subtraction. Alternatively, a timer may be started wheneither the reference pulse or the rotor pulse occurs, and end when theother of the reference pulse and the rotor pulse occurs, resulting inthe time between the rotor pulse and the reference pulse. For eachgenerator, during OCC, the time between the rotor pulse and a referencepoint (either positive-slope or negative-slope) is measured 1030, andthe period of rotation for this time is measured and recorded.Alternatively, and as discussed above and in conjunction with FIG. 9A, areference point on a signal waveform other than a zero crossing (such asa sample time) may be used. The generators are then connected to a loadsuch that power is generated 1040. The rotor pulse and the referencepoint or reference time can be time stamped 1045. The time between therotor pulse and the reference point or reference time is measured 1050,and the period of rotation is also measured and recorded. As above, inthe example where the reference point or time and the rotor pulse aretime stamped, the time between the reference point or time and the rotorpulse may be calculated using subtraction. Alternatively, a timer may bestarted when either the reference point or time or the rotor pulseoccurs, and end when the other of the reference point or time and therotor pulse occurs, resulting in the time between the rotor pulse andthe reference point or time. The power angles δ_(p) for each generatorare then calculated using the recorded times and periods of rotation1060. As described below, the power angle may be time stamped with acommon time and shared across a network 1070. Further, the time-stamp ofthe rotor pulse, the time stamp of the reference point or time may alsobe shared over the network.

Thus, the change of the power angles of each generator with reference tothe reference time or point may be monitored and used to make powersystem decisions. For example, if a zero crossing from generator A 902in FIG. 9A is used as the reference point, then the power angle ofgenerator A 902 may be calculated and monitored, and the power angles ofeach of generators B 904, C 906, and D 908, relative to generator A 902may be calculated and monitored. When the power angle of any of thegenerators relative to generator A indicates a problem on the electricpower system 900, actions may be taken such as shedding loads,disconnecting generators, and the like.

While specific embodiments and applications of the disclosure have beenillustrated and described, it is to be understood that the disclosure isnot limited to the precise configuration and components disclosedherein. Various modifications, changes, and variations apparent to thoseof skill in the art may be made in the arrangement, operation, anddetails of the methods and systems of the disclosure without departingfrom the spirit and scope of the disclosure.

For example, though the rotor displacement monitor described herein isdescribed as being used on an electrical power generator, it is to beunderstood that the monitor may be used to monitor the angular rotationof several types of equipment with rotors such as, for example, motors,transmissions, drills, and the like. Several rotor displacement monitorsmay be used to monitor angular positions of a single rotor at variouspoints along the rotor. For example, a transmission rotor may have alength such that axial torsion occurs when a mechanical angularresistance is placed on the rotor. A rotor displacement monitor of thepresent invention may be placed at both ends of the rotor to monitor thedegree of angular torsion in the rotor.

The present power angle monitor can further be used to calculate a powersystem frequency. This system uses a mechanical means (the rotor anglemonitor) to calculate the rotational speed of a particular generator.For example, for a synchronous generator the electrical frequency (inHz) can be calculated using the number of poles and the rotational speedof the rotor:

Frequency of a signal waveform (such as a current or a voltage waveform)may be used by various power system protection, control, monitoring andautomation schemes. The frequency as calculated using the system hereinmay be used in these schemes. For example, the actual power systemfrequency is used in the correction of voltage or current phasors wherethe signal waveform is sampled at a rate corresponding with a frequencythat is different from the actual frequency of the signal waveform, asdiscussed in U.S. Pat. No. 6,934,654. Further, the actual signalwaveform frequency is needed for resampling a waveform signal formeasuring voltage or current phasors. For example, U.S. Pat. Nos.6,662,124 and 6,845,333 describe methods and systems including afrequency estimation circuit. The system and method of the presentinvention could be used in place of the frequency estimation circuit.

1. A system for monitoring the angular displacement of a rotor of anelectrical power generator, comprising: an electric power generatorcomprising a rotor including a marking, and terminals in electricalcommunication with the generator, to provide a signal waveform from thegenerator; a light source to direct a beam of light toward the rotor; anoptical sensor adapted to detect reflections of the beam of light fromthe rotor; and, a controller in electrical communication with theterminals and the optical sensor to generate a marking profile includinga record of a signal from the detected reflections of the beam of light,and to generate a rotor pulse when the optical sensor detects areflection corresponding with the marking.
 2. The system of claim 1,wherein the controller comprises a time input and is adapted todetermine a period of rotation of the rotor using the time input and therotor pulse.
 3. The system of claim 1, wherein the controller comprisesa time input and is adapted to record a first time between the rotorpulse and a reference point on the signal waveform received from theterminals.
 4. The system of claim 3, wherein the reference pointcomprises a zero-crossing of the signal waveform.
 5. The system of claim3, wherein the controller is adapted to record a first time between therotor pulse and reference point for an open circuit condition, and thecontroller is adapted to record a second time for a second state whereload is connected to the generator.
 6. The system of claim 5, whereinthe controller is adapted to calculate a power angle of the electricpower generator using the period of rotation, the first time, and thesecond time.
 7. The system of claim 5, wherein the controller is furtheradapted to: calculate a first period of rotation corresponding with thefirst time; calculate a second period of rotation corresponding with thesecond time; and calculate a power angle of the electric power generatorusing the first and second periods of rotation and the first and secondtimes.
 8. The system of claim 1, further comprising a common timesource, and wherein the controller includes a common time source inputand is adapted to time stamp the power angle using the common timesource input.
 9. The system of claim 3, wherein the controller furthercomprises a common time source input, and wherein the controller isadapted to calculate an angle of the signal waveform, time stamp thereference point, the rotor pulse, and the angle.
 10. The system of claim1, further comprising a network for communicating power angles from aplurality of generators.
 11. The system of claim 1, wherein the markingcomprises a scratch on the surface of the rotor.
 12. The system of claim1, wherein the marking comprises an intentionally placed mark.
 13. Thesystem of claim 1, wherein the controller is adapted to generate therotor pulse when the marking is detected in spite of additional markingon the rotor.
 14. The system of claim 1, wherein the light sourcecomprises one selected from the group consisting of: a laser, a lightemitting diode generating visible light, a light emitting diodegenerating invisible light, and combinations thereof.
 15. A method fordetecting rotations of an electric generator with a rotating rotor thatincludes a reference mark thereon, comprising the steps of: directing abeam of light toward the rotor such that the beam of light may bereflected from the rotor; generating a marking profile including arecord of a signal from detected reflections from the beam of lightreflected from the rotor; detecting the reference mark on the rotorusing light reflected from the rotor; and generating a rotor pulse whenthe reference mark is detected.
 16. The method of claim 15, wherein thestep of detecting the reference mark on the rotor further comprises thestep of recognizing a plurality of markings on the rotor including thereference mark.
 17. The method of claim 16, further comprisingdetermining a period of rotation of the rotor using the rotor pulse. 18.The method of claim 15, further comprising the step of detecting a newmark on the rotor and updating the marking profile accordingly.
 19. Themethod of claim 15, comprising the further steps of: measuring a firstperiod of rotation of the rotor when no load is connected to thegenerator; calculating a first time between the rotor pulse and areference point when no load is connected to the generator: measuring asecond period of rotation of the rotor when a load is connected to thegenerator; calculating a second time between the rotor pulse and areference point when a load is connected to the generator; andcalculating a power angle of the generator using the first and secondperiods of rotation and first and second times.
 20. The method of claim15, further comprising the step of placing the marking on the rotor. 21.The method of claim 19, further comprising the step of assigning a timestamp to the power angle based on an input from a common time source.22. The method of claim 19, further comprising the steps of: gatheringpower angles from a plurality of generators connected to a network; and,making a power system protection, control, or automation decision basedon a comparison of the gathered power angles.
 23. The method of claim19, further comprising the steps of: calculating an angle of signalwaveform generated by the generator; assigning a time stamp to theangle; assigning a time stamp to the rotor pulse; assigning a time stampto the reference point of the signal waveform; and communicating thetime stamped angle, rotor pulse and reference point to a communicationsnetwork.
 24. The method of claim 19, wherein the reference point is apoint on a signal waveform generated by the generator.
 25. The method ofclaim 24, wherein the point is a zero crossing of the signal waveformgenerated by the generator.
 26. The method of claim 19, wherein thereference point is selected from the group consisting of: a point on asignal waveform generated by the generator; a point on a signal waveformgenerated by another generator; a reoccuring point in time; a samplesignal on a network; a rotor pulse from another generator; andcombinations thereof.